The technology of fabricating and manufacturing primary or secondary battery cells has been around for many years. The electrochemistry and the principals of how rechargeable and non-rechargeable batteries work have been thoroughly and exhaustively reported in many books and publications. Similarly the material of construction for the electrodes, separator, type of electrolyte, and the cell design parameters for the currently available commercial batteries have been thoroughly investigated. In general, a battery cell consists of two electrodes, positive and negative, a separator to avoid internal short circuiting, and electrolyte, typically in liquid or gel form. The chemical reaction at the electrodes is the source of electron generation that can be transferred from one electrode to the other through an external circuit. In rechargeable batteries, this electrochemical reaction is reversible. i.e., with an external electron source such as a charger, the active material of the electrodes can be reclaimed. Typical rechargeable battery designs are given in "Rechargeable Batteries Application Handbook ", EDN series, 1992. An extensive review of battery cell technology can be found in Kirk-Othmer Encyclopedia Of Chemical Technology, Wiley. New York.
Commercially available battery cells contain electrode plates on which the active material of the electrodes are deposited. Many different methods of depositing the active material on the electrode plates have been utilized. These methods consist of simple impregnation on a porous substrate, electrochemical plating, sputtering, and many other form of coating techniques. The substrates chosen for depositing the active material, usually have high surface area, such as highly porous surfaces, polymeric foams with open structure, fiber mats, etc. The high surface area of the substrate results in availability of more active sites on each electrode that participates in the electrochemical reaction and generation of electrons, hence increasing the cell capacity. The active material used in construction of the positive and negative electrodes consist of but not limited to nickel, nickel oxy- hydroxide, cadmium, lead, lead oxide, silver, mercury, zinc, oxygen, hydrogen, graphite, nickel-lanthanum alloys, etc. in crystalline or amorphous form. If the substrate is not electrically conductive, it may in turn be deposited or attached to an electrically conductive material typically referred to as current collector. For the purpose of clarity the combination of the active material, the substrate, and the current collector is referred to as the electrode from here thereafter. In commercially available battery cells, the electrode plates are electrically isolated from each other by another plate of a porous material so that an internal short-circuit does not occur. The separator used is typically a porous polymeric material with good wetting characteristics or of the glass fiber type. The media between the two electrodes is filled with an electrolyte solution such as sodium or potassium hydroxide, sulfuric acid or other alkaline and acid solutions. The porous structure of the separator allows for ion transfer between the positive and negative electrode and free diffusion of gases generated at the electrodes. In design of certain sealed rechargeable battery cells, the capacity of the active material of one electrode must be controlled such that full charge is achieved preferentially on one electrode before the other, in order to avoid evolution of excessive gases. Also, the amount of electrolyte used is only enough to wet the surface of the two electrode plates as a thin film and fill the pores of the porous separator material.
The purpose of the separator plate between the two electrodes in a battery cell is to allow minimal space between the two electrodes and hence maximize the amount or the surface area of the electrode that can be packed into a battery container of certain size. In certain commercially available batteries, the electrode plates are fabricated into strip of flat plates which along with a strip of separator material are wound together into a roll configuration and inserted into a container.
It is an object of the present invention to provide a structure for the configuration of the two electrodes and the separator such that a single fiber or two fibers in contact with each other will make up a single cell or micro-cell. The geometry of the cells of this invention provide an extremely high surface area of the electrodes per unit volume when numerous fibrous cells are packed into a given container size. The extremely high surface area of the electrodes exposed to the electrochemical reaction translates into extremely high energy density per volume.
It is a further object of this invention to provide a process for inserting a fibrous electrode inside a porous insulator or forming a thin layer of a separator material with porous structure around a fibrous electrode.